Identifying the existence and the location of an unknown quantity of radioactive material becomes more difficult when the source is intentionally shielded to hid characteristic radiation. With shielding, only the high energy gamma rays will emerge for possible detection. Unfortunately these same high energy gamma rays are hard to detect with full energy deposition making collection of the energy spectra inefficient. A system is desired that can both identify and locate a shielded source of unknown type and quantity using only a few detected gamma rays.
In addition to the control of unauthorized movement of radioactive material in production and storage sites this system could be deployed to monitor contraband radioactive material in large staging and storage sites. Identification of possible dirty bomb material or strategic nuclear material, including nuclear weapons, at docks, airports and other transportation and shipping hubs is possible.
The identification of the illicit movement of radioactive materials requires a system capable of identifying the material and locating the source. The ability to shield contraband radioactive material from detection render most energy spectrum based identification and localization systems ineffective. Activation methods require separation and isolation of suspected containers into a radiation hazardous environment. Innocuous monitoring of movement in open staging or storage areas is highly desirable but difficult to achieve.
Conventional monitoring techniques involve detection of emitted characteristic radiation, the identification of the energy of those emitted radiations and correlation of the measured energy or energies to the known emissions spectra of candidate radioisotopes. Various imaging techniques can be used in conjunction with the energy spectroscopy including physical collimation of the incident radiation, coded aperture arrays and conventional Compton cameras.
Detection methods such as gamma ray spectroscopy are sensitive and highly developed for determining the presence and radionuclide composition of radioactive materials. Gamma ray spectra are therefore useful for identifying a radioactive material, but not for locating it. Current methods for locating radioactive sources typically rely on collimators or intensity measurements at several positions, and require long data acquisition times due to low counts when the radioactive sources are shielded.